Five-axis machines provide an ability to translate a tool or workpiece along three orthogonal axes (X, Y and Z), and to tilt the tool about a fourth and fifth orthogonal axes. With the ability to tilt the tool, five-axis machine tools are valuable in the production of machined parts. For example, these five-axis machine tools allow a user to perform multiple cutting, abrading or drilling operations on a piece of material without having to stop operations and mount different specialized tools. This eliminates the extra time involved in setting up each specialized tool and the extra time involved in mounting the tool to the five-axis machine. Thus, the workpieces can be converted into machined parts faster and cheaper.
The ability to tilt a tool relative to a workpiece is particularly advantageous in machining workpieces with an abrasive water jet of particles fired from a nozzle at high speeds, in machining with a laser, and in painting. FIG. 1A illustrates an abrasive water jet 12 cutting a workpiece 14, and a resulting deflection distance L in the jet 12 in a direction opposite of jet motion 19. Every abrasive water jet application is affected to some extent by the deflection 18 of the abrasive water jet 12 stream from a longitudinal axis 54 as the nozzle 10 moves across the workpiece 14 in a direction indicated by motion 19. The faster the nozzle 10 moves, the more the abrasive water jet 12 is bent by the structure of the workpiece away from longitudinal axis 54. When the motion 19 of the nozzle 10 is a straight line, the abrasive water jet 12 stream cuts the material of the workpiece 14 the way a wheel cutter might cut with the stream exiting the bottom of the workpiece 14 at the deflection distance L behind the place of impact 13 where the water jet 12 stream enters the workpiece 14. On straight cuts, the stream 12 can be moved swiftly across the workpiece 14 because the stream's deflection 18 is directly inline with and behind the place of impact 13, and does not affect cutting accuracy. However, on corners, the deflection distance L can cause cutting errors as it flares to the outside or inside of a corner leaving behind or cutting undesirable deflection tapers.
Every abrasive water jet application is also affect by a bevel taper in the cut edges of the workpiece 14. FIG. 1B illustrates a bevel taper 20 in the cut edges 22a and 22b of the workpiece 14 formed by the jet 12. The jet 12 is truncated in FIG. 1B for clarity. Jet cutting, particularly with an abrasive water jet, typically produces undesirable tapered or beveled cut edges 22a and 22b in a workpiece. The widest portion of the bevel taper 18 is typically toward the place of impact 13. The bevel taper 18 looks much like a sharpened end of a pencil was dragged through the workpiece 14. The bevel taper 20 is function of material thickness, and is generally greatest in thin material where the bevel taper 20 may be 10 degrees. In thicker material such as two-inch steel, the bevel taper 20 is much less, though still significant. The bevel taper 20 is also a function of cutting speed. The bevel taper 20 becomes less as cutting speed slows, and then as cutting speed further slows beyond a point, the bevel taper 20 reverses from that illustrated in FIG. 1B becoming narrower toward the point of impact 13. The bevel taper 20 typically can only effectively be eliminated by tilting the nozzle 10 relative to the workpiece surface 15 along the X-axis. Determination of the amount of tilt required in a particular application to eliminate a bevel taper is not part of the claimed invention.
Unlike the bevel taper 20, a deflection taper may be reduced by slowing the motion 19 of the nozzle 10 across the workpiece 14. To cut complex shapes with a variety of corners and curves, the traverse speed of the motion 19 must be constantly adjusted. In addition, reducing undesirable deflection tapers requires that the abrasive water jet 12 continues removing material from the cut surfaces 16 even after the abrasive water jet 12 has penetrated the thickness of the workpiece 14. Another method of reducing undesirable deflection tapers is to make multiple passes with the abrasive water jet 12 across the workpiece 14. These methods increase time necessary to cut the workpiece 14.
An ability to tilt the nozzle 10 relative to the workpiece surface 15 provides advantages for jet cutting. For straight-line cutting, the nozzle 10 and abrasive water jet 12 can be orientated normal to the workpiece surface 15 with a compensation tilt along the X-axis to minimize the bevel taper 20 because deflection taper is not an issue. Undesirable deflection tapers in corners can be reduced by additionally tilting the nozzle 10. Alternatively, the speed of the abrasive jet's movement 19 across the workpiece 14 can be maintained in a first cut with only the compensation tilt to minimize the bevel taper 20, and then a subsequent cutting pass made across the workpiece 14 with the nozzle 10 additionally tilted to remove the deflection taper produced in the previous cutting pass. This can be quicker than making one slow cutting pass that does not produce deflection tapers.
Abrasive water-jet cutting obtains other benefits from tilting a tool 10 relative to the workpiece surface 15. For example, when the tool 10 turns an inside corner, the abrasive water jet 12 is deflected into the workpiece at the deflection distance L as the abrasive water jet 12 begins to move out of the corner. This can be minimized by beginning the movement with a tilt of the abrasive water jet 12 in the direction of movement 19 to advance only the bottom of the jet until the head and nozzle 10 can be moved in the new direction without being deflected into the workpiece. Thus, to efficiently accomplish high-speed cutting with preferred angles on the cut surfaces, the nozzle 10 may be mounted to a five-axis machine which has two orthogonal axes of horizontal translation (X and Y), one axis of vertical translation (Z), and two orthogonal axes of tilt at the nozzle 10.
Unfortunately, five-axis machines are typically expensive and can tie up a significant amount of working capital. A reason for this expense is the requirement that a five-axis machine allows cutting, grinding or drilling within very close dimensional tolerances while bearing high loads are frequently encountered in machining workpieces. The high loads result from the forced contact of the grinding wheel, drill bit, or saw blade against the workpiece. Because these conventional tools for removing material from a workpiece are mounted to the tool and require contact with the workpiece to perform, the tool must bear the loads encountered during their operation. Consequently, conventional five-axis machine tools must be robust and, thus, are typically expensive to manufacture.
When machining a workpiece with an abrasive water jet, a five-axis machine does not have to bear high loads during the cutting, grinding, or drilling process because the nozzle 10 does not contact the workpiece. Instead, the five-axis machine needs to bear the reaction load of the abrasive water jet 12 being expelled from the nozzle 10 at high speeds. This reaction load is typically much lower than the forced contact load generated by conventional grinding wheels, drill bits, or saw blades and it can be kept relatively constant during the cutting process. Consequently, the robust nature of a conventional five-axis machine is not required to operate an abrasive jet tool.
Thus, there is a need for an inexpensive apparatus for holding and tilting a tool that provides a user the ability to pivot a tool relative to a workpiece about two orthogonal working axes.